Props should not be mutated by Plot component

It seems like the Plot component mutates its props (at least layout) instead of cloning the data.
Mutation of props is to me unexpected (and undesired) behavior for a react component.

If for instance the layout is stored in the state of a component and then fed to the Plot component as a prop, like

render() {
  <Plot
    layout={this.state.layout},
    ...
    />

we will have a problem, since this.state only should be updated with the react setState() method.

该 issue 指出了 Plot 组件及其 layout prop；首先定位组件对 layout 的处理，并使用由 state 保存的 layout 重现该行为。当组件不再修改传入的 layout，并通过回归测试完成验证时，即表示完成。